14 DAY TRIAL // Most of the oceans' algae protect themselves using chemical "sunscreens" against the sun's ultraviolet rays. These are pigments associated with colorful surfaces.
A recent study has proved that monocellular algae called coccolitophores use microscopic patterns of tiny calcium carbonate crystals from their shell as protector against UV rays while allowing longer wavelengths through for photosynthesis. The cell shell of calcite crystals looks like a microscopic piece of macram?.
To understand better how these structures reflect light, Rafael Quintero-Torres of the National Autonomous University of Mexico and his team made a disk-like structure imitating the coccolitophore shell to see how this functions. The disk had a two-layered structure: the bottom layer contains spherical reflectors in a "honeycomb" arrangement of hexagons, while in the top layer the spheres form a sparser pattern of triangles.
The microcrystals were approximated as spheres because, at 90 nm diameter, they are too small for the relevant wavelengths of light to "notice" the box-like shape they take on in some coccolithophore species. The reflectivity of the entire structure is determined by the combined effect of light reflecting from each sphere. Different light waves reflected within the structure can enhance or partially cancel one another, depending on the wavelength. The double layer structure becomes sharply more reflective for wavelengths less than about 400 nm. Light waves longer than 400 nm - which enter in the visible specter the cell uses for photosynthesis - is more easily transmitted.
The results were similar for light reaching the surface at any angle and with different polarizations. "UV reflection at all angles is important for survival, says Quintero-Torres, because algae in the oceans have all possible orientations."
If the spherical crystals were placed randomly on the structure, they showed a slight tendency to reflect visible wavelengths more efficiently than ultraviolet. This confirms that the ordered arrangement of spheres is critical to the ultraviolet reflectivity. "Biological designs such as this also hold lessons for applied physicists," says Quintero-Torres.
Scientists have been developing photonic crystals, highly ordered microstructures with reflecting and transmitting properties, which can manipulate light signals just as silicon chips manipulate electronic signals. "In looking for new ways to design photonic crystals, says Quintero-Torres, we can learn from nature how to solve problems."
Similarly ordered photonic crystals generate the colored patterns of butterfly wings, beetle carapace, peacock feathers.